Priority Korean Patent Application Nos. 2001-2243 filed on Jan. 15, 2001 and 2001-32749 filed on Jun. 12, 2001, are incorporated herein in their entirety by reference.
The invention relates to a method for producing a dot-printless light guide plate for a liquid crystal display device, and more particularly a method for producing a dot-printless light guide plate for a liquid crystal display device in which the dot-printless light guide plate is made of addition polymers consisting of norbornene-based monomer and olefin-based monomer. The addition polymers used on the present invention have a good transmission rate of light between 400 nm to 800 nm of frequency and do not have a double bond in their chemical structures so that yellowness does not occur. The polymers also have an advantage that their melt flow rate and heat resistance can be controlled to a desirable degree by changing substituents of ethylene and norbornene. Therefore, they are excellent in moldability for a dot-printless light guide plate and coating composition containing cesium compound or silicon compound can be readily applied to them. Further, the light guide plate made of the polymers has a good bending resistance due to its low hygroscopicity.
Back lighting for liquid crystal display devices commonly used can be divided into two types. One is a side-light type in which a cold cathode florescent tube is disposed in the lateral side of a light guide plate and the other is a multi-lamp type in which two or four lamps are disposed in the lateral side of by a light guide plate in order to enhance luminance of a back lighting apparatus.
The light guide plate according to the present invention is for using as a component of the back lighting apparatus of the side-light type. An illuminating device of the side-light type is disclosed in Japanese Patent Laid-Open Publication No. 57-128383. This illuminating device has a structure in which a light source such as a cold cathode gas discharge tube, a hot cathode gas discharge tube, a light bulb or LED is disposed on the lateral side of a light emission surface. The light sources can be adapted in various shapes, for example, L-shape, U-shape, W-shape, etc., depending on their use to be applied. In the illuminating device, the light emitted from a light source is introduced into a light guide plate through a lateral side of the light guide plate, and transferred via a light diffusion plate and a polarizing plate into observing part by changing it""s advancing direction using a light scattering device disposed on a surface of the light reflecting surface.
The back lighting apparatus of the side-light type having a light source on the lateral side can contribute to reduction of the overall weight and thickness of a liquid crystal display device. Therefore, recently, they are used as an illuminator for liquid crystal display devices in a laptop, a notebook, a personal computer, etc. Such portable electronic equipments as a notebook are driven by a built-in battery and thus require a low power consumption of the illuminating device of the side-light type. For example, in case of a notebook, the back lighting apparatus consumes 60% of power. In order to reduce the power consumption, it is desired to enhance light transmission efficiency of material for forming the light guide plate, the diffusion plate, the polarizing plate, etc by improving transparency and regularities of luminance of them.
Further, back lighting apparatus composes 60% of the overall thickness of liquid crystal display devices in portable equipments. Accordingly, it is desirable to lighten and thin the light guide plate for achieving compactness of the portable equipments.
FIG. 1 shows an example of the side-light type illuminating device. A liquid crystal panel 8 creates character or image information by controlling light transmittance on a desired position of a screen. The liquid crystal panel 8 is not light-emissive and supplied with light from the illuminating part. A cold cathode florescent tube that consumes relatively less power, is commonly used as a light source 1. A light guide plate 5 has a light emission surface and a sloped rear surface opposed thereto. Although the light guide plate 5 appears in a wedge shape in FIG. 1, it may be flat or any particular irregular shape. In addition to the light guide plate 5, various sheets having supplementary functions such as a reflective plate 3, diffusion plate 6, polarizing plate 7, etc., are stacked on the light guide plate 5.
In the general light guide plate, a light scattering pattern 4 is formed by dot-printing method with white ink on the rear surface of the light emission surface to improve light emitting efficiency as shown in FIG. 1. However, the process for forming the light scattering pattern 4 by the dot-printing method with white ink has defects as follows.
In the process for forming a light scattering pattern using white ink, the pattern is printed poorly as it becomes minute, so light reflection effect of the pattern is reduced. Further, the luminance of the print is damaged, for example, due to discoloration, as time goes by, and consequently, the lifetime of the illuminating device will be shortened.
In order to solve the above-described problems, a dot-printless light guide plate has been developed. For example, U.S. Pat. No. 6,123,431 discloses a dot-printless light guide plate having grooves thereon to form a light scattering pattern. Also, U.S. Pat. No. 5,881,201 discloses a dot-printless light guide plate in which organic or inorganic particles are dispersed, which have a different refractive index compared to the basic resin. Thus, the light guide plate has the light scattering function by variations in refractive index throughout the light guide and serves as a light diffusion plate as well. However, in order to form the light scattering pattern by means of molding in the process for preparing the dot-printless light guide plate, it is needed that the base resin should have a good melt flow index and heat resistance.
Japanese Patent Laid-Open Publication No. 10-265530 and U.S. Pat. No. 5,883,163 disclose a method for producing a light guide plate with an excellent formability using polymethylmethacrylate. However, since PMMA has a high melt viscosity and a low fluidity, it is difficult to mold a thin light guide plate in a large size. When the process temperature is raised to increase the fluidity, the resin foams in the cylinder, which may cause voids in the molded article. Further, PMMA has a relatively high specific gravity of 1.2. So, it can hardly satisfy the requirements of thinness and lightness for the light guide plate used in liquid crystal display devices of portable equipments. PMMA also tends to be deformed due to its high hygroscopicity and hence, a large-sized light guide plate is easily bended.
Meanwhile, polycarbonate (herein after referred to as PC) is suggested as an optical material, for example, as an optical disk in Japanese Patent Laid-Open Publication No. 9-183894 but it cannot be easily applied to the light guide plate. PC cannot provide a sufficient luminance as a back lighting since it has a high birefringence value. Further, though PC has a heat deformation temperature higher than that of PMMA, it requires an increased molding temperature to improve the fluidity for molding. However, under the condition of high molding temperature, PC is likely to be foamed or hydrolyzed by absorbing moisture. For this reason, it is difficult to produce a thin molded article in a large size and with a good mechanical strength through an injection molding. In addition, it is difficult to form a micro-pattern on the rear surface of the light guide plate with PC.
Japanese Patent Laid-Open Publication No. 9-296028 suggests a hydrogenated norbornene polymer as a novel optical material, which has a specific gravity lower than that of PMMA and a good heat resistance. However, this norbornene resin is produced by complex ring opening polymerization and hydrogen treatment. Also, there are several problems in forming a thin plate in a large-size for using as a light guide plate. Since the resin of the above invention has a high hygroscopicity of 0.24, bending may occur in the produced thin article. Further, the fluidity of the resin is poor due to its high molecular weight. Moreover, the resin may be yellowed by discoloration resulting from oxidation, which leads to deterioration of whiteness related to the luminance of the light guide plate.
A feature of the present invention is to apply an addition polymerized norbornene copolymer to produce a light guide plate.
In accordance with the feature of the present invention, there is provided a method for producing a dot-printless light guide plate for a liquid crystal display device using an addition polymerized norbornene copolymer represented by the following general formula (1): 
wherein R1, R2, R3 and R4 are respectively a hydrogen atom, a C1xcx9cC10 linear, branched or cyclic alkyl group, or xe2x80x94COOR7 in which R7 is a C1xcx9cC10 linear, branched or cyclic alkyl group; R5 and R6 are respectively a hydrogen atom or a C1xcx9cC10 linear, branched or cyclic alkyl group; and x is an integer of 0 to 4.